The reverse recovery behavior of a diode is shown in the graph presented in FIG. 1A. Initially, the diode is conducting current in the forward direction. Once the reverse recovery is initiated, the forward current begins to decrease, and ultimately begins flowing in the reverse direction. The reverse current increases during the time ta until it reaches the maximum reverse recovery current Irm. Thereafter, during the period of time tb the reverse current decreases until it returns to approximately zero. At this point the diode is able to block the reverse flow of current. The reverse current flow during ta and tb allows the diode to remove the charge that has built up in the device while it is turned on in the forward state. The total amount of charge that needs to be removed (the reverse recovery charge Qrr is the shaded area under the curve. Therefore, in order to decrease the recovery time it is desirable to decrease both the value of Irm, and Qrr. However, if the reverse current falls too sharply towards the end of the reverse recovery period, stray circuit inductance may cause an increased voltage across the device. The softness value, S, is a measure of tb/ta, and is useful in determining if the stray circuit inductance will be too large. Typically, devices with a softness value greater than 1.0 will not have problems with stray circuit inductance causing harm to the device during the reverse recovery.
The Qrr of a diode is largely controlled by the injection efficiency of the device. A diode with a high injection efficiency will generally have a higher Qrr. Prior art attempts to reduce the Qrr have therefore focused on reducing the injection efficiency. The injection efficiency may be reduced by decreasing the carrier lifetime in the diode. Specifically, this may be accomplished by processing steps such as, electron radiation, proton radiation, helium irradiation and/or gold or platinum diffusion into the silicon of the diode. However, these processes also lead to increased leakage current in the diode and degraded reverse recovery performance at high temperature.
U.S. patent application Ser. No. 12/931,429, filed Jan. 31, 2011 (U.S. Patent Application Publication No. 2012193676A1) and incorporated herein in its entirety, describes several alternative techniques for reducing the injection efficiency. An example of such a diode is shown in FIG. 1B. First, the top P-layer 109 is lightly doped. The light doping reduces the injection efficiency from the top side of the device because there are fewer charge carriers available. The injection efficiency from the top side may be further reduced by adding a highly doped N-barrier layer 108 immediately below the top P-layer 109. Additionally, the injection efficiency from the bottom of the device is reduced by removing the semiconductor substrate from the device. By way of example, the semiconductor substrate may be removed by back grinding. However, the extent to which the doping concentration of the top P-layer 109 is reduced is limited by the punch through constraint and the quality of the ohmic contact between the contact metal 112 and the upper P-layer 109. Therefore, there is a need in the art to improve the reverse recovery performance by lowering the injection efficiency while still maintaining a good ohmic contact to the upper P-layer 109 and a low leakage current.
It is within this context that embodiments of the present invention arise.